Summary:
A bottle falls into the ocean and is tossed about during the credits.
Doug McClure's narration tells us it was June 3rd, 1916 when
a U-boat fired on their ship. He and Lisa Clayton drift on a
lifeboat until they join another boat of surviving officers.
When the U-boat doesn't see them and surfaces, they wait for the
Germans on top at the hatch, fight, and take over.

They fail to signal a British warship successfully
and it attacks them, but they submerge. The Germans tamper with
the compass so that they travel south instead of west to North
America, and the Germans eventually regain command of the U-boat.
The German Captain talks with Lisa, who turns out to be a biologist
and who is incredulous that the Captain can have such eclectic
interests. He explains: "Life is formed on killing and
destruction."

Lisa releases the prisoners and they once again
gain control of the boat by blowing up a German ship which was
being signalled. But soon they are lost in the ice floes. The
German Captain pontificates about an 18th-century Italian explorer
seeing no way into the uncharted land "Caprona." They
find warm water and deduce the existence of an underground river.

The moment they emerge in "the land that
time forgot," they are attacked, first by a dinosaur mouth
lunging at the periscope, then when a plesiosaur eats one of the
crew. They shoot at this animal until, with enough bullets in
its mouth and neck, it fall to the deck. We cut to a steaming
platter of food--the dinosaur ("eat or be eaten"
dynamics)--served
to the anxious humans.

We accustom ourselves. Bacteria in the water
is a problem, natives wage battles occasionally, and dinosaurs
lumber (but are killed in a reptilian-nervous-system's delayed-reaction
manner). A native they capture helps them find oil, but what
with a bloody allosaur/triceratops battle, we have to "keep
an eye on those monsters." After the triceratops wins, the
boat is able to fire long-distance on it and kill it.

We're all cooperating to haul oil for the escape
from this land, where as one follows the "river north, the
more highly developed they [the creatures] became." A fight
breaks out, instigated by the German First Mate, who we don't
trust now as we do the Captain. A pterodactyl snatches our native
friend and flies away. Lisa is temporarily captured by natives
until a volcano sends earthquakes and firestorms about. During
a quicksand rescue and while some dinos are burning, the other
humans are climbing aboard the U-boat. The First Mate mutinies
against the Captain, who is shot and wounded, before he himself
is killed, as the U-boat submerges to leave, with Tyler and Lisa
on the shore abandoned. But the volcano has made the water too
hot and the U-boat blows up.

The narration returns, as does the image of
the bottle falling into the sea, as Tyler explains that they travel
"ever northward" in the snow, but hell, he'd rather
be here with Lisa than anywhere else without her.

Commentary:
The juxtaposition of Germans and the insistence on the superiority
of the "north" is complex but interesting. Best, though,
is the illustration of the "eat or be eaten" dynamics
the moment they emerge in "the land that time forgot."
Naturally (?), they prevail and this is expressed in their dining
on dino.